Portable wireless communications devices have received scrutiny regarding their safety with respect to the potential danger associated with the transmission of the signals from such apparatus. When a user of a wireless communication device, such as a cellular telephone, talks on the device, he holds the telephone up his head so that the earpiece is in contact with his ear. In close proximity is the antenna, which usually extends from the top surface of the telephone and which transmits electromagnetic radiation. Typically, the antenna of cellular phones and other wireless communications technologies (PCS, G3 or Blue Tooth) emit radiation in the UHF and/or microwave frequency ranges.
The effect of this electromagnetic radiation on the tissues of the user is being studied. Investigations are underway attempting to ascertain whether links exist between this radiation and maladies such as cancer, weakening of the blood-brain barrier, and high blood pressure. (see, Cellular Phones: Why the Health Risk Can't be Dismissed, Microwave News, January/February 1993; Digital Mobile Phone Radiation Causes Rise in Blood Pressure, Microwave News, July/August 1998; Questions and Answers About Electric and Magnetic Fields Associated with the Use of Electric Power, National Institute of Environmental Health Sciences, U.S. Department of Energy, November 1994.) As public awareness of the potential health risk has grown, so too has the demand for reducing the amount of radiation directed toward and absorbed by the user. Additionally, undesired electromagnetic radiation has also been found to cause interference in certain sensitive electronic equipment located nearby. FIG. 3 shows a typical configuration for a cellular phone 1, wherein a telescoping or fixed antenna element 5 is disposed externally from the top surface of the phone. The figure also shows lines 3 representing electromagnetic radiation that are emitted from such an antenna. Often these types of antennas result in an asymmetric radiation pattern because the shape and dimensions of the ground plane of a printed wire board (PWB) (incorporating the phone's circuitry) that is used as a counterpoise for the antenna element results in an unequal current distribution in the antenna element and in the ground plane.
In general, electromagnetic wave propagation has been controlled in commercial and military applications as a means to reduce signal jamming at certain locations, to locate targets, or to enhance gain and directionality in desired areas. Past approaches to radiation reduction have utilized several art forms, including the use of shields made by special materials, or other means such as the use of multiple radiating or parasitic elements within a symmetrical or asymmetrical dipole antenna configuration. Typically, the size and distance between radiating elements, along with other variables, offers a means to create the desired wave pattern. These approaches are unconcerned with, and produce inconsistent results for, electromagnetic wave propagation near the user's antenna array and head.
It is known in the art that by providing shielding, some undesired electromagnetic radiation may be suppressed. This approach is taken by Luxon, et al., in U.S. Pat. No. 5,666,125, and Humbert, et al., in U.S. Pat. No. 5,124,889.
Others have attempted to control of electromagnetic wave propagation by employing symmetrical or asymmetrical antenna configurations (Uda-Yagi approach). U.S. Pat. No. 6,147,653 to Wallace, et al., describes a balanced dipole antenna for a mobile phone comprised of a radiator element and counterpoise electrically isolated from the PWB of the mobile phone. As controlling directivity in the far-field, rather than reducting electromagnetic energy near the antenna arrays was the goal of those inventors, the antenna elements are geometrically arranged in such a manner as to create a uniform gain in the azimuth. U.S. Pat. No. 6,239,765 to Johnson, et al., describes utilizing an asymmetric dipole antenna assembly for communications devices operating at predetermined wavelengths and having a transceiver circuit, conductor trace elements plated onto a dielectric using common printed circuit board manufacturing technology with the traces having a first end, a one-quarter wavelength electrical length and a second dipole half. Some communications engineers, however, are skeptical of using directional configurations in the industry. See “Handset Antennas and Humans”, IEEE Proceedings, January 1995.
A third approach to controlling electromagnetic wave propagation has been to employ an array wherein signals generated are phased (in or out) or the signals are cross-polarized. For example, U.S. Pat. No. 6,292,135, to Takatori, et al., describes an adaptive array antenna designed to identify and strengthen or weaken desired signal strengths in poor multipath environments. And U.S. Pat. No. 6,275,199 to Chen describes a nulling direct radiating array and a plurality of auxiliary arrays symmetrically disposed about the main array. This system includes a nulling processor, an adaptive weighting network and weight generator within the nulling processor, and is related to a military application of blocking jamming signals again originating far from the passive receiving antenna array system, rather than reducing radiation emitted from a wireless device.
Accordingly, a need exists for an antenna array for use with a wireless communications device, wherein the antenna array is configured and excited in a manner that will reduce or eliminate undesired electromagnetic radiation near the antenna array.